Supercontinuum lasers delivering continuous spectra over an ultra-broad bandwidth offer an ideal source for many applications within biomedical imaging, component characterisation, manufacturing control and defense research. The most common form of supercontinuum laser comprises an ultrashort pulse optical pump source operating at a pump wavelength in the Infra-Red (IR) region of the spectrum (typically around 800 nm for a Ti:Sapphire laser or 1064 nm for mode-locked fiber lasers and diode pumped solid-state lasers) and a highly nonlinear photonic crystal fiber, with specially designed dispersion properties. The interaction between the high intensity optical pump pulse and the nonlinear silica fiber causes extreme broadening into the visible and infra red regions of the spectrum to provide spectra spanning from approximately 450 nanometers (nm) to beyond 2.5 micrometer (μm).
The phenomenon of supercontinuum was first proposed in the 1970's by Alfano et al (see R. R. Alfano and S. L. Shapiro, “Observation of self-phase modulation and small-scale filaments in crystals and glasses”, Phys. Rev. Lett. 24, 592 (1970)) and in 2000 the first demonstration of a fiber-based supercontinuum made by Ranka and Windelar (see J. K. Ranka et al., “Visible continuum generation in air-silica microstructure optical fibers with anomalous dispersion at 800 nm”, Opt. Lett. 25 (1), 25 (2000); see also U.S. Pat. No. 6,097,870) using a Ti:Sapphire mode-locked femtosecond laser 10 to pump a photonic crystal fiber 11 with zero dispersion wavelength at approximately 760 nm in free space through launch optics 12 as shown in FIG. 1. More recently, researchers and companies have utilized nanosecond and picosecond-based pump sources operable at around 1064 nm wavelength to pump photonic crystal fibers with a zero dispersion wavelength close to 1 μm to generate high brightness supercontinuum products.
Commercial supercontinuum products, such as Fianium Limited's supercontinuum fiber laser model no. SC450, rely on mode-locked fiber oscillators and high-power optical fiber amplifiers to generate highly intense pulses at the pump wavelength, which when injected into a nonlinear photonic crystal fiber (PCF), result in extreme spectral broadening into both the visible and IR regions of the spectrum.
FIG. 2 shows an example of such a system, where a mode-locked fiber oscillator 21 produces low energy optical pulses of approximately 10 picoseconds (ps) duration and at a repetition rate of 20 MHz to 100 MHz. The pulses are amplified within a cascaded fiber amplifier 22, comprising one or more stages of amplification, with optical isolation 23 between each stage. The output of the amplifier is a high-energy pulse of up to 400 nJ and tens of kilowatt peak power. The output of the amplifier is injected into a length of highly nonlinear photonic crystal fiber (PCF) 24 with anomalous dispersion at the pump wavelength (1064 nm) and zero dispersion at a wavelength close to the pump wavelength. The spectral bandwidth of the pulse broadens within the PCF, creating a pulse of approximately 100 nJ energy and a bandwidth from 400 nm to 2.5 μm that exits the PCF 24.
The pulsed nature of these sources and high repetition rates (20 MHz to 160 MHz), makes these sources attractive to both applications requiring quasi-continuous wave radiation as well as applications involving time-resolved measurements such as FLIM (Fluorescent lifetime imaging) and TCSPC (time correlated single photon counting). However, often, the high repetition rates can be a limiting factor in lifetime imaging, where the pulse-to-pulse separation (50 ns for a 20 MHz source, down to a few nanoseconds for a 160 MHz source) can be much shorter than the lifetime of the sample under evaluation. In this event, one requires a lower pulse repetition rate.
In general, most mode-locked laser systems operate at pulse repetition rates of several tens of MHz (diode-pumped solid-state (DPSS) lasers typically operate from 80 MHz to 100 MHz, and fiber lasers from 20 MHz to several hundreds of MHz). Some applications require lower repetition rates—for example fluorescent-imaging lifetime microscopy (FLIM), where fluorescent-labeled molecules are excited by an incident optical pulse and the decay of the fluorescence is monitored before the next optical pulse arrives. In FLIM, the lifetimes of interest can very often exceed tens or even hundreds of nanoseconds, and therefore it is often required to have repetition rates of 40 MHz and below. Repetition rates of less than 20 MHz, while not impossible to achieve, are difficult to deliver from a mode-locked fiber oscillator due to high nonlinearity within a long cavity (10 m for a 10 MHz oscillator). From a DPSS source, such a cavity (5 m long for 20 MHz and 100 m in length for 1 MHz) is almost impossible to make due to the required complexity of the cavity design.